This invention relates to a method of and apparatus for the manufacture of slab-stock polymeric foam, such as polyurethane foam of the type used in seats, cushions and the like.
In the production of polyurethane foam reactants, in particular surfactants, catalysts, blowing agents and modifiers, are thoroughly mixed and deposited into a mold. The reactants are polyol, isocyanate, for example toluene di-isocyanate (TDI), and water. Polyol reacts with the TDI to form urethane linkages that build the polymer. TDI reacts with water to produce carbon dioxide which blows the foam. Additional water lowers the density of the foam and also increases exothermic heat output. This heat limits water blown foams to a density of 19.2 Kg/m.sup.3 at an atmospheric pressure of 1 bar.
At this density temperatures inside large foam buns will reach or exceed 177 degrees C. and remains high for several hours--the foam acting as its own insulation. These high temperatures create the possibility of auto oxidation leading to scorching or even fire.
Inert blowing agents such as CFC-11 (also known as chlorofluorocarbon-11 or fluorotrichloromethane) or methylene chloride are used to produce lower density and softer foams. These materials soften the foam by reducing the number of hard segments of the polymer caused by water for any given density. The other active ingredients are a silicone surfactant which stabilizes the gas bubbles and permits efficient use of the foam blowing gases, a polymer building catalyst such as stannous octoate, a gas producing catalyst that catalyzes the isocyanatewater reaction such as an amine compound, and various modifiers such as colours, anti-static agents and combustion modifiers.
The mould into which the expanding foam is introduced takes the form of a channel with stationary or moving sidewalls 122 cm. to 152 cm. high and a bottom conveyor usually about 244 cm. wide. One or both the sidewalls may be adjustable so that variable width foam buns can be made.
Foam is deposited onto the bottom of the channel by means of a lay-down assembly including a trough having a volume of approximately 100 litres. As the foam expands, it spills out of the trough into the channel.
Foam expansion takes approximately 75 to 180 seconds depending on the foam formulation. The foam rises freely in the production channel until it reaches maximum height. As the foam cells break open they release the gases of expansion which are hot from the exothermic reaction heat. These gases are mainly carbon dioxide and the inert blowing agents CFC-11 and methylene chloride. These hot gases pull small amounts of TDI vapours and possibly some of the other components with them. TDI is a dangerous respiratory irritant and regulations in the USA at least require a maximum exposure of 0.02 parts per million in the workplace.
To meet this requirement and to minimize CFC-11 and methylene chloride vapours, foam plants typically have extensive high capacity ventilation systems that expel large volumes of air from the foam production areas. It is difficult to get all of these chemical vapours out of the plant so that workers are exposed to acceptable levels of these chemical vapours. Even so, exhausting these chemicals into the atmosphere is now suspected of causing injury to the environment, for example by damaging the ozone layer of the atmosphere, and by causing smog.